ex21_12.cpp

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/* Code for exercise 21.12.
 | 
 | "C++ Solutions--Companion to The C++ Programming Language, Third Edition"
 | by David Vandevoorde; Addison-Wesley 1998; ISBN 0-201-30965-3.
 |
 | Permission to use, copy, modify, distribute and sell this software
 | and its documentation for any purpose is hereby granted without fee.
 | The author makes no representations about the suitability of this
 | software for any purpose.  It is provided "as is" without express or
 | implied warranty.
 `----------------------------------------------------------------------*/

#include <fstream>
#include <iostream>
#include <stdlib.h>


struct MappedFile {
   MappedFile(std::ifstream &file): file_(file) {}
   char operator[](std::ifstream::off_type i) const {
      file_.seekg(i);
      return char(file_.get());
   }
private:
   std::ifstream &file_;
};


void error(char const *prog, char const *entity, char const *msg) {
   std::cerr << prog << ": " << entity << msg << '\n';
   exit(1);
}


int main(int argc, char *argv[]) {
   if (argc!=2)
      error(argv[0], "", ": unexpected number of arguments.");
   std::ifstream pass0(argv[1]);
   if (!pass0)
      error(argv[0], argv[1], ": cannot open file for reading.");
   std::ifstream::off_type n = 0;
   int chksum0 = 0;
   while (pass0) {
      chksum0 += char(pass0.get());
      ++n;
   }
   std::cerr << "Reading " << n << " characters.\n";
   std::cerr << chksum0 << '\n';
   int chksum1 = 0;
   std::ifstream pass1(argv[1]);
   if (!pass1)
      error(argv[0], argv[1], ": cannot open file for reading.");
   MappedFile accessor(pass1);
   for (size_t k = 0; k!=n; ++k) {
      chksum1 += accessor[k];
   }
   std::cerr << chksum1 << '\n';
   return 0;
}